The objective of this paper is to introduce a new approach to the evolutionary design of polymorphic digital circuits at the transistor level. A discrete event-driven simulator was utilized to achieve reasonable trade-off between performance and precision. The proposed approach was evaluated on a set of polymorphic logic circuits controlled by switching the power rails. It was demonstrated that the proposed method is able to produce valid solutions. A lot of polymorphic gates based on ambipolar transistors were designed, which provide transistor savings compared to existing circuits. A new class of polymorphic gates was discovered due to the proposed system - gates based on conventional MOS transistors whose functions are changed by switching the power rails. They seems to have the best parameters from currently known polymorphic gates based on conventional transistors.